Responses Of Droughts To Climate Change And Hazard Propagation

As one of the most devastating natural disasters,droughts have been characterized by complicated mechanisms and long persistentence,associated with widespread impacts and catastrophic consequences,and have imposed considerable economic,societal and environmental challenges across the world.Since the second industrial revolution,the greenhouse gas emissions have soared.The warming atmosphere has altered the global energy budget,impacted the substance circulation processes,and accelerated the hydrological cycle,exacerbating the uneven spatiotemporal distribution of water resources and leading to more frequent droughts.As a typical drought-prone country,China has suffered from frequent,persistent,widespread and severe drought hazards,which have caused dire socioeconomic losses and environmental damages.Therefore,this study investigates the spatiaotemporal variations of hydrometeorological fluxes and their themodynamical responses to climate warming,elaborates the drought hazard transferability from meteorological to hydrological propagation,identifies the contribution of internal climate variability to future drought variations.Further,we also investigate the responses of drought hazards to 1.5℃and2.0℃warming levels from the global perspective and quantify the corresponding socioeconomic exposures.The main research works and findings are summarized as follows:（1）The research goal and significance of investigating the responses of drought propagation and hazard variations under climate change are clarified.The definition,methodology and assessment frameworks of droughts are briefly reviewed and discussed,and the up-to-date advancement and development about climate change impacts on droughts are summarized.（2）Based on a multi-model ensemble outputs consisting of 31 climate models,the high-resolution（0.5°×0.5°）gridded meteorological dataset and the daily streamflow series of 151 catchments over China,we employ the DBC method and four conceptual lumped hydrological models（XAJ,GR4J,HBV,HMETS）to obtain the simulated daily precipitation,temperature and streamflow series covering 1961-2100 period over 151 catchments over China.Then the spatialtemporal variations of basin-scale precipitation,temperature and streamflow are investigated.The themodynamic responses of these hydrometeorological fluxes to climate warming are also quantified.The results show that the annual precipitation is projected to increase at the national scale,accompanied by some seasonality and spatial heterogeneity.The increasing changes can also be observed in annual streamflow projections,with more promoninent seasonality and spatial heterogeneity.Both the precipitation and streamflow are highly sensitive to the rising temperature and show super C-C scaling rates,with slightly higher scaling rates in precipitation than in streamflow.The scaling types between precipitation and streamflow are similar to some extent.The northeastern and southwestern catchments generally show the monotonic increasing scaling type,while the central and southern catchments in China typically demonstrate the hook sturcture type.（3）The reponses of droughts to climate warming and the drought hazard transferability from meteorological to hydrological propagation are investigated.The results show that the“wetting”tendency observed in the atmosphere and landsurface does not indicate drought mitigation.On the other side,the water fluxes’themodynamic responses to the rising temperature are closely related to future drought variations.In detail,the monotonic increasing scaling type indicates future drought mitigation;while the hook structure or the monotonic decreasing type implies future drought deterioration.When droughts propagating from the climates to the terrestrial part of the water cycle,the water deficits tend to amplify,resulting in lengthening and more severe drought episodes.Also,the spatial heterogeneity of hydrological drought variations is more prominent than that of meteorological droughts.Moreover,the responses of hydrological droughts to climate warming are projected to be stronger than those of meteorological droughts.（4）The CESM1 involving 40 members and CSIRO consisting 10 scenarios are employed to estimate the internal climate variability（ICV）impacts on droughts.The multi-model ensemble is used to estimate the anthropogenic climate change signals（ACC）and inter-model uncertainty（IMU）,which are served as comparisons to measure the contribution of ICV.Generally,the contribution of ICV is the largest in drought duration,followed by drought frequency and drought severity.In the middle of 21^st century,ICV plays a dominant role in climate change uncertainty.When considering the ICV impacts on future drought variations,more frequency droughts with lengthening duration and ascending severity are projected over majority regions of China in the middle of the 21^st century;while these conditions are projected to be slightly mitigated in the end of the 21^st century,with soaring model uncertainty.（5）Based on three RCP scenarios（RCP2.6、RCP4.5、RCP8.5）and 13 climate models,the 1.5℃and 2.0℃warming levels are determined.Then the tendencies of global climates and the responses of global droughts are projected under different warming levels.The most scenarios are incorporated to the Copula theory to investigate the bivariate drought hazards and corresponding population and GDP exposures under the 1.5℃and 2.0℃warming levels.The results demonstate that the middle latitude regions and the tropics（i.e.the Mediterranean,South Africa,southern North America and South America）are projected to suffer significantly deteriorated drought hazards.In the 1.5℃warming target,more than 75（73）countries’ population（GDP）will be completely affected by increasing drought risks,while an extra 0.5℃warming will further lead to an additional 17 countries suffering from a nearly unbearable situation.Our results demonstrate that limiting global warming to 1.5℃,compared with 2℃warming,can perceptibly mitigate the drought impacts over major regions of the world.